142 THE ANATOMY OF THE HORSE. 



The spinal cord begins at the foramen magnum by continuity with 

 the medulla oblongata, and it is here of considerable thickness. Trac- 

 ing it backwards, it is seen to become thicker behind the middle of 

 the cervical region, forming the cervical enlargement, which extends as 

 far as the 2nd dorsal vertebra. It is from this enlargement that the 

 nerves which supply the fore limb are given off. Beyond the 2nd 

 dorsal vertebra the cord contracts slightly, so as to become about the 

 middle of the back smaller even than in its initial portion. Preserving 

 this diminished thickness throughout the dorsal region, it again expands 

 in the lumbar region, forming a second swelling — the lumbar enlarge- 

 ment, from w T hich the nerves for the supply of the hind limb are de- 

 tached. Beyond the lumbar enlargement the cord rapidly becomes 

 reduced in volume,, and tapers to a point about the 2nd sacral seg- 

 ment. This tapering extremity of the cord — the conus medullaris — is 

 prolonged backwards by the filum terminale, into which its nervous 

 structure is continued for a little distance. 



The Spinal Nerves of the horse number forty-two or forty-three pairs, 

 and their number in the different regions of the vertebral column is 

 expressed in the following formula : — 



The 1st cervical nerve leaves the canal by the antero-internal foramen 

 of the atlas, the 2nd by the foramen in the front of the arch of 

 the axis, and the others in succession pass out by the intervertebral 

 foramina. 



In the other regions the nerves are numbered according to the 

 vertebrae behind which they emerge ; thus, the 1st dorsal nerve emerges 

 by the intervertebral foramen behind the 1st dorsal vertebra, and so on 

 with the others. 



In the cervical region the nerves pass nearly directly outwards from 

 the cord to their points of exit from the canal. In the dorsal region, 

 however, it will be observed that each nerve is slightly inclined back- 

 wards from the side of the cord to the foramen by which it emerges. In 

 the lumbar region this backward inclination of the nerves becomes 

 augmented, and it continues to increase in the same way in each nerve 

 of the sacral and coccygeal regions. The sacral nerves thus have their 

 roots detached from the lumbar part of the cord, while the coccygeal 

 nerves are given off by the terminal part of the cord, which, as already 

 stated, does not extend beyond the middle of the sacrum These last 

 nerves have therefore a length of several inches within the spinal canal ; 

 and as they pass back together, each to reach its aperture of exit, they 

 have an arrangement which resembles the hairs of a horse's tail, and is 

 therefore termed the caucla equina. 



Roots of the Nerves. Each spinal nerve has two roots connecting 

 it with the spinal cord — a superior and an inferior. The superior^ 



